Is it because we are stressed that our muscles stiffen up or is it that our muscles stiffen up, for whatever reason, and then, we feel stress?

It definitely seems to be that the road goes both ways! Grassi and Passatore, in 1988, published on the role of the sympathetic system, on skeletal muscles.

More traditionally, it has been considered that the sympathetic nervous system is involved in vegetative functions. That being said, some evidence point to the fact that it could modulate some somatic inputs by acting upon receptor organs such as mammalian mechanoreceptors.

It was at the beginning of the century that controversy concerning the existence of an action exerted by the sympathetic nervous system upon skeletal muscles and unrelated to vasomotor activity.

In fact, there are two strategies for the participation of the sympathetic nervous system in motor control:

As far as the first mechanism goes, it was shown that administration of adrenaline or stimulation of the sympathetic nervous system contributed to a 10%-20% in tension developed under isometric conditions.

As well, there is consideration at this point for the fact that the sympathetic nervous system could play a role in the sensitization of muscle spindles. Over the years, irrefutable anatomical evidence of sympathetic innervation of muscle spindles has been shown.

Based on the anatomy and physiology presented here, it is clear that one cannot separate stress from muscle activity. It seems to stress the importance of managing muscle behavior holistically, addressing any component that could create an imbalance between the parasympathetic and sympathetic components of the autonomic nervous system!